The present invention relates to a method of crosslinking polysilazane polymers in which boron modified silazanes are used as crosslinkers. The invention also relates to a novel crosslinker comprising boron modified tris(trimethylsilylamino)silane and a method for it manufacture.
A variety of polysilazane oligomers, cyclics, resins and linear polymers are known in the art. Such polysilazanes are characterized as having backbones with alternating silicon and nitrogen atoms. These polymers have found broad utility as precursors to a variety of ceramic materials such as ceramic monoliths, ceramic fibers and matrices for ceramic matrix composites.
To be truly useful as ceramic precursors, however, the polysilazanes must be curable (infusible) to prevent deformation of the ceramic upon heating. Various approaches to providing curability have been suggested. For instance, Mahone in U.S. Pat. No. 5,086,126 discloses a process for adding vinyl groups to a polysilazane such that upon addition of a free radical precursor the polymer would rapidly cure.
Similarly, various references disclose the addition of boron compounds to polysilazanes to provide curability. For instance, Zank in U.S. Pat. No. 5,169,908 discloses the addition of borane to a hydridopolysilazane polymer to render the polymer curable. Funayama et al. in U.S. Pat. No. 5,030,744 discloses the addition of a boron compound to a polysilazane to increase its molecular weight. U.S. Pat. No. 4,910,173 granted to Niebylski discloses the formation of an organoborosilazane by the reaction of a boroxine with a polysilazane. Seyferth et al. in J. Am. Ceram. Soc. 73, 2131-2133 (1990) teaches the reaction of a silazane oligomer with borane to form a higher molecular weight borazine. Finally, Noth in Z. Naturforsch, B. Anorg. Chem. Org. Chem. 16 [9] 618-621 (1961) teaches the reaction of hexamethyldisilazane with diborane to form a higher molecular weight borazine.
As is readily apparent, each of the above references teaches the addition of boron to a polysilazane to render it infusible or to increase the molecular weight of the resultant polymer. By contrast, the present inventors have now discovered that boron-modified silazane crosslinkers can be utilized to render nearly any polysilazane having Si--H or N--H bonds infusible.